Add a modifiers uv project with python
The value you are trying to set is a collection so you have to choose an index as you can have several projectors, use:

bpy.ops.object.modifier_add(type='UV_PROJECT')
bpy.context.object.modifiers["UVProject"].projectors[0].object = bpy.data.objects["Camera"]


See bpy.types.UVProjectModifier.